Off-axis three-mirror anastignat having corrector mirror

ABSTRACT

A corrector mirror folds the optical path between the objective and relay portions of a three-mirror anastigmat. The corrector mirror is a non-powered mirror having a nominally flat but higher order aspheric surface. By placing the corrector mirror between the objective portion and an intermediate image formed by the objective portion, the field offset of the anastigmat can be significantly increased. A large field offset makes the off-axis anastigmat ideal for use with an on-axis dewar for infrared imaging applications.

This is a divisional application Ser. No. 08/247,362, filed May 23, 1994now abandoned.

BACKGROUND OF THE INVENTION

This invention relates in general to all-reflective optical systems andin particular to an off-axis three-mirror anastigmat.

Reflective optical systems have long been the champion of theastronomical community, primarily because of their size, lightweightconstruction and broad spectral coverage. Slowly gaining popularity inother communities, reflective optical systems are now beginning tochallenge the established refractive optical systems.

In general, reflective optical systems provide superior performance overrefractive optical systems. Reflective optical systems provide superiorthermal stability and radiation resistance, and they offer lower imagedefects arising from chromatic aberration (unlike reflective elements,refractive elements focus different wavelengths of radiation atdifferent focal points).

For certain applications, reflective optical systems can be made farmore compact than refractive systems. Reflective systems can operate ona wider range of wavelengths than can refractive optics withoutintroducing geometric aberrations and distortion. A reflective opticalsystem can operate on both visible and infrared radiation. In contrast,an all-refractive system can operate on visible light or it can operateon infrared radiation, but it cannot operate on both visible andinfrared radiation. Thus, an all-reflective surveillance camera wouldrequire only a single set of optics for viewing visible and infraredradiation, whereas an all-refractive camera would require two sets ofoptics: one set for viewing visible radiation, and the other set forviewing infrared radiation. The size and weight savings are impressiveand obvious; the elimination of boresight issues is equally impressive,but less obvious.

One type of all-reflective system having a wide range of applications isa three-mirror anastigmat (TMA). The TMA is a re-imaging system, havingan objective portion that forms an intermediate image and a relayportion that relays the intermediate image to a plane for viewing. TheTMA permits correction of the three fundamental types of geometricaberrations: spherical aberration, coma and astigmatism (three mirrorsbeing the minimum number of elements required for correction of theseaberrations in the absence of certain symmetry conditions). The TMA canalso be designed to correct for curvature of the field of view.

One such TMA 2 is shown in FIG. 1. The TMA 2 includes a primary mirror3, a secondary mirror 4, and a tertiary mirror 5. The primary mirror 3receives optical signals through an entrance pupil 6 and forms anintermediate image 7, which is between the primary mirror 3 and thesecondary mirror 4. The secondary mirror 4 and tertiary mirror 5cooperate to relay the intermediate image through an exit pupil 8 to afocal plane 9 for viewing. This TMA 2 is disclosed in Cook U.S. Pat. No.4,834,517, issued on May 30, 1989 and assigned to Hughes AircraftCompany, the assignee of this invention. Cook U.S. Pat. No. 4,834,517 isincorporated herein by reference.

The off-axis TMA 2 covers wide fields of view on a flat focal surface atfast optical speeds (optical speed, denoted by an f/number, isproportional to the amount of light collected by the optical system, andit can be calculated as the angle of the F-cone or equivalently as thefocal length of the optical system divided by the entrance pupildiameter). For tactical infrared imaging, the off-axis nature of the TMA2 yields an unobscured aperture, and the relayed nature allows strayradiation to be rejected. The relayed nature of the TMA 2 also allowsfor 100 percent cold shielding, which is critical for modern tacticalinfrared detectors.

In addition to the above beneficial characteristics, the TMA 2 has anadditional characteristic that can be valued quite highly. Due to thesignificant angle at which the imaging F-cones intercept the focal plane9, the TMA 2 can be designed to preclude the reflection of radiationback to its source. This overcomes a problem known as signatureaugmentation, a phenomenon which is apparent to anyone who has taken aphotograph of a person with a camera having its flash bulb mounteddirectly above the camera's lens: the person in the picture appears tohave "red eyes." Signature augmentation occurs because the retinaabsorbs all but red light from the bulb, and reflects the red light backto the camera lens and onto the film. If the TMA 2 is operated at asmall incidence angle, it too will reflect light back to the lightsource. In certain wide-field applications, this can have seriousconsequences.

It is apparent from FIG. 1 that the elimination of signatureaugmentation requires the imaging F-cones to be everywhere outside thenormal of the focal plane 9. This necessarily offsets the exit pupil 8from the focal plane 9, thereby requiring an off-axis cryo-dewar to bebuilt. For those applications where the presence of signatureaugmentation is of no concern, the cost of the off-axis dewar presents ahardship.

The TMA 2 could accommodate an on-axis dewar (where the cold shieldaperture is directly over the focal plane array) if the field of viewwere offset signficantly. However, such a large field offset would notallow the correction of image aberrations and distortion to the levelsgenerally required for most applications.

SUMMARY OF THE INVENTION

Solutions to the problems above are provided by a three-mirroranastigmat comprising an objective portion operative to form anintermediate image; a corrector mirror, disposed in an optical pathbetween the objective portion and the intermediate image; and a relayportion for relaying the intermediate image. For those applicationswhere the augmentation signature is not a concern, and where an on-axisdewar is greatly desired, the addition of the corrector mirror improvesupon the optical form shown in FIG. 1. The corrector mirror allows thefield of view to be offset sufficiently, providing substantially on-axisF-cones, while still allowing for the desired correction of imageaberrations and distortion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a ray-trace section of a three-mirror anastigmataccording to the prior art;

FIG. 2 illustrates a ray-trace section of a three-mirror anastigmataccording to this invention; and

FIG. 3 illustrates an infrared detection system employing thethree-mirror anastigmat of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 illustrates a TMA 10 including a primary mirror 12, secondarymirror 14 and tertiary mirror 16, all disposed substantially about acommon axis C. The primary mirror 12 receives optical signals 18 throughan entrance pupil 20 and forms an intermediate image 22. The secondaryand tertiary mirrors 14 and 16 cooperate to relay the intermediate image22 through an exit pupil 24 to a focal plane 25 for viewing.

The TMA 10 also includes a corrector mirror 26, a nominally flat mirror,for folding the optical path between the primary and secondary mirrors12 and 14. The corrector mirror is located between the primary mirror 12and the intermediate image 22. The corrector mirror 26 allows the fieldoffset to be increased, allowing the TMA 10 to be operated further offaxis than the TMA 2 of FIG. 1. While increasing the field offset, thecorrector mirror 26 does not degrade the performance of the TMA 10; widefield of view, image quality and distortion correction are maintained.

The power distribution of the primary, secondary and tertiary mirrors12, 14 and 16 is positive, negative, and positive. The corrector mirror26 is preferably non-powered, but could have a slight positive ornegative power, and it could have a higher order aspheric figure. Theprecise powers of the mirrors 12, 14, 16 and 26 are selected such thatthe sum of the powers is zero. A zero sum provides correction of thePetzval sum, causing a zero curvature in the focal plane (i.e., asubstantially flat field condition).

The cross-section of the primary mirror 12 can be conic (e.g.,paraboloid, hyperboloid or ellipsoid) or a higher order aspheric. Thecross-section of the secondary mirror 14 is typically hyperboloid, butcan also be a higher order aspheric. The cross-section of the tertiarymirror 16 is typically ellipsoid, but can also be a higher orderaspheric. The corrector mirror 26, though nominally flat, is generallyprovided with an aspheric surface. The aspheric surface also allows thecorrector mirror 26 to reduce aberrations in the pupil imagery. Thesegeometries are left up to the discretion of the optical designer.

The mirrors 12, 14, 16 and 26 can be designed on a computer with aray-tracing software package. Sag (z) of each of the mirrors 12, 14, 16and 26 can be determined by the following equation, which is an industrystandard: ##EQU1## where C=1/radius; D, E, F and G are constants;

ρ² is the radial distance on the mirror; and

κ is a conic constant=-(eccentricity)².

From this equation, a prescription for the TMA 10 can be generated. Onesuch prescription is shown in the Tables below. The TMA prescribed inthe Tables has excellent image quality and distortion correction over a6×8 degree field of view at a speed of f/4. It must be stressed,however, that the prescription in Tables I and II is merely exemplary,and that the prescription of each TMA is determined by the intendedapplication. Therefore, TMAs for different applications will havedifferent prescriptions.

                                      TABLE I                                     __________________________________________________________________________    Sufface                                                                             Radius                                                                             κ                                                                            D      E      F      G      Thickness                         __________________________________________________________________________    Entrance                                                                            ∞                                                                            --   --     --     --     --     5.404                             Pupil 20                                                                      Primary                                                                             -8.881                                                                             -0.95795                                                                           -0.32653 ×                                                                     0.97780 ×                                                                      -0.62631                                                                             0.18665 ×                                                                      -3.604                            Mirror 12       10.sup.-4                                                                            10.sup.-5                                                                            10.sup.-6                                                                            10.sup.-7                                Corrector                                                                           -18.808                                                                            --   0.15005 ×                                                                      -0.43172 ×                                                                     0.80245 ×                                                                      -0.64804 ×                                                                     1.869                             Mirror 26       10.sup.-1                                                                            10.sup.-2                                                                            10.sup.-3                                                                            10.sup.-4                                Second                                                                              2.758                                                                              1.6575                                                                             0.41085 ×                                                                      -0.72084 ×                                                                     0.21828                                                                              -0.23068                                                                             -2.330                            Mirror 14       10.sup.-1                                                                            10.sup.-1                                              Tertiary                                                                            3.244                                                                              -0.05388                                                                           0.28958 ×                                                                      0.54620 ×                                                                      -0.30259 ×                                                                     0.11991 ×                                                                      2.853                             Mirror 16       10.sup.-3                                                                            10.sup.-4                                                                            10.sup.-5                                                                            10.sup.-5                                Exit  ∞                                                                            --   --     --     --     --     0.836                             Pupil 24                                                                      Focal ∞                                                                            --   --     --     --     --     --                                Plane 25                                                                      __________________________________________________________________________     (+) Radii have centers to the right;                                          (+) Thicknesses are to the right;                                             (+) Tilts are counterclockwise; and                                           (+) Decenters are up and are performed before tilts.                     

                  TABLE II                                                        ______________________________________                                        Effective focal length, inch                                                                        3.60                                                    Entrance aperture diameter, inch                                                                    0.90                                                    F-number              F/4.0                                                   Field of view, deg                                                            Elevation             6.0                                                     Azimuth               8.0                                                     Entrance aperture offset, inch                                                                      2.107                                                   Field of view offset, deg                                                                           7.5                                                     ______________________________________                                    

Composition of the mirrors 12, 14, 16 and 26 is dependent upon theapplication for which the TMA 10 is intended. For wavelengths in thevisible spectrum, the mirrors 12, 14, 16 and 26 can be made of materialssuch as glass, metal, plastic or advanced composite. For wavelengths inthe infrared spectrum, the mirrors 12, 14, 16 and 26 can be made ofmaterials such as glass, plastic, metal or advanced composite. Themethod of fabricating the mirrors 12, 14, 16 and 26 is dependent uponthe composition. Fabrication processes include conventional polishing,computer-controlled polishing, precision machining, replication andmolding.

When being assembled, the mirrors 12, 14, 16 and 26 can be aligned bybeing bolted together (typically for precision-machined mirrors) orsnapped together (typically for plastics). The method of alignment isdependent upon the composition of the mirrors 12, 14, 16 and 26, themethod of their fabrication, and the intended application.

Thus disclosed is a compact, re-imaging all-reflective optical systemthat is especially suited for wide field of view applications (eitherline fields for scanning systems or two-dimensional fields for staringsystems) where the focal cones for the center of the field must besubstantially normally incident on the focal plane 24.

FIG. 3 shows an infrared imaging system 28 which takes advantage of theoff-axis operation of the TMA 10. The system 28 includes the TMA 10 andan on-axis dewar 30 having a cold shield 32 centered directly above adetector array 34. A cold finger 36 forms a thermal connection betweenthe detector array 34 and a cryogenic source (not shown). The correctormirror 26 of the TMA 10 does not have detrimental packaging effects andcan even allow more favorable configurations in some instances.

The TMA 10 of the infrared imaging system 28 can be provided with afield stop (not shown) located between the primary and secondary mirrors12 and 14 to permit passage of the intermediate image, while blockingthe passage of substantially all stray electromagnetic radiation outsideof the field of view. Failure to block this stray radiation could resultin high levels of noise and spurious signals which degrade the abilityof the detector array 34 to detect infrared radiation.

It should be noted that the TMA 2 of FIG. 1 cannot be used with anon-axis dewar 30 because the dewar 30 would interfere with the incominglight. Therefore, the only other feasible combination would be anon-axis TMA (e.g., TMA 2) with an off-axis dewar. However, the on-axisnature of dewar 32, that is, the cold shield 32 being centered directlyabove the detector 34, makes the on-axis dewar 30 less complex and,therefore, more desirable than the off-axis dewar, whose cold shield isnot centered directly above the detector array.

It will be understood that the embodiments described herein are merelyexemplary and that a person skilled in the art may make many variationsand modifications without departing from the spirit and scope of theinvention. For example, a corrector mirror could be placed between thesecondary mirror and intermediate image of the three mirror anastigmatdisclosed in Cook U.S. Pat. No. 4,265,510. All such modifications areintended to be included within the scope of the invention as defined inthe appended claims.

I claim:
 1. An all-reflective optical system for receiving and focusingoptical signals comprising:(a) a primary mirror having an optical axisand a concave reflecting surface yielding significant net positiveoptical power and operable to create a subsequent intermediate image;(b) a real unobscured entrance pupil displaced from said optical axisand located prior to said primary mirror; (c) a significant twodimensional field of view displaced from said optical axis that issubstantially in focus at said intermediate image; (d) a low powercorrector mirror having a general aspheric reflecting surface andpositioned between said primary mirror and said intermediate image; (e)a secondary mirror having a convex reflecting surface yieldingsignificant net negative optical power and positioned subsequent to saidintermediate image; (f) a tertiary mirror having a concave reflectivesurface yielding significant net positive optical power and positionedsubsequent to said secondary mirror; (g) a final image surface that issubstantially flat and located subsequent to said tertiary mirror and isoptically conjugate to said intermediate image by the operation of saidsecondary and tertiary mirrors; and (h) a real unobscured exit pupildisplaced from said optical axis and positioned between said tertiarymirror and said final image surface and optically conjugate to saidentrance pupil by the operation of said primary, corrector, secondary,and tertiary mirrors.
 2. The optical system of claim 1 wherein said exitpupil is substantially centered on said final image surface such that animaging f-cone at the center of said image is substantiallyperpendicular to said image surface.
 3. The optical system of claim 1,wherein said primary, secondary, and tertiary mirrors have conic sectionconfigurations.
 4. The optical system of claim 1, wherein said primary,secondary, and tertiary mirrors have general aspheric configurations.